Get applicable specs working with Cassandra 1.2/CQL3

* Define constant CQL_VERSION in specs, read from environment and
  defaults to 2.0.0
* Use this constant to filter specs which have a :cql_version option
  defined
* Set :cql_version to 2.0.0 for specs that deal with comparators and
  sparse columns
* Create spec helper modules to deal with different setup behavior for
  CQL2 and CQL3, particularly introspecting for the existence of a
  column family
* In some specs, needed to explicitly define columns which could be
  lazily created by insert in CQL2